RPM stands for Revolutions per minute in terms of a hard disk (a some other products too)
In short, it tells you the access speed of the hard disk.
It is used to help determine the access time on computer hard drives. RPM is a measurement of how many complete revolutions a computer's hard drive makes in a single minute. The higher the RPM, the faster the data will be accessed; for example, if you were comparing two hard drives, one with 5400 RPM and another with 7200 RPM, the hard drive with a 7200 RPM will be capable of accessing data much faster than the 5400 RPM drive. However, it is also important to note that a 5400 RPM drive will be much cheaper than a 7200 RPM drive.
While it may not be a noticeable speed difference when loading small files, the RPM of a hard drive can make a dramatic difference when loading large files or several hundred or thousand files. We always recommend users get at least a 7200 RPM drive when buying a traditional hard disk drive.